A flywheel for energy storage and recovery is typically mounted on a shaft in a housing such that the flywheel and the shaft move substantially together. The shaft is mounted on the housing via a bearing arrangement which allows the shaft (and thus the flywheel) to rotate relative to the housing. Kinetic energy can be stored in the flywheel by increasing the flywheel's speed of rotation. Energy can be recovered from the flywheel by transferring the flywheel's rotational energy to another entity, such as a vehicle drive shaft.
A flywheel which rotates at high speeds (e.g. speeds in excess of 15,000 rpm or 20,000 rpm) experiences large air resistance (or ‘windage’) forces, because of the high tip speed of the flywheel. This leads to loss of kinetic energy from the flywheel. To reduce such losses, a vacuum pump may be used to at least partially evacuate a chamber within the housing, the flywheel being positioned within the chamber. Evacuating the chamber lowers the pressure within the chamber, optimally to approximately a vacuum level, meaning that the flywheel experiences less air resistance as it rotates in the chamber.
In order for the chamber within the housing to be evacuated and the pressure within the chamber to be maintained at a low level, a seal must be provided between the walls of the housing and the shaft, to prevent (or at least reduce) ingress of fluids such as air or bearing lubricant into the evacuated chamber. The seal must remain effective even when the shaft is rotating at high speed relative to the housing. However, the seal should introduce as little resistance to rotation of the flywheel (e.g. through friction between the seal and the shaft) as possible or the benefit of evacuating the chamber will be lost.
The flywheel and the housing may in some circumstances undergo translational movement relative to one another (in addition to the intended rotational movement relative to each other). Translational movement of the flywheel and the housing relative to each other may include, for example, vibrational motion of the flywheel and the shaft within the housing.
Relative translational motion of the flywheel and the housing can be problematic for several reasons. Firstly, translational movement of the shaft relative to the housing makes it difficult to maintain a seal between the shaft and the walls of the housing, meaning that fluids such as air and oil can pass the seal and enter the evacuated chamber, raising the pressure within the chamber. Secondly, translational motion (e.g. vibration) of the flywheel and the shaft within the housing may cause translational motion (e.g. vibration) of the housing itself relative to whatever body the housing is mounted on or in. Vibration can be loud and/or physically unsettling, and therefore unpleasant for users of the flywheel apparatus. It can also have undesirable consequences if parts of the flywheel and/or the housing come loose due to the vibration. These problems may be particularly bad if one or more of the vibrating bodies achieves a resonant mode of vibration.
The invention aims to overcome or at least ameliorate the problems discussed above.